Traditionally, intermediate frequency (IF) sections have been employed in receivers to perform the major portion of a radio's selectivity since it may be technically difficult or cost prohibitive to develop sufficiently selective filters at the received frequency. The process of translating a signal down to zero Hertz intermediate frequency is known as zero-IF downconversion.
One of the problems associated with implementation of zero-IF downconverters is that of amplitude and phase imbalance in the quadrature oscillator and mixers. This results in audio distortion. FIG. 1A shows a simple model of a zero-IF circuit, which takes a real input signal Re{x(t)e.sup.j.omega.t } at 101, centered at radian frequency .omega., and translates the signal to zero Hertz, producing the complex signal x(t). The circuit mixes the input signal separately by cos (.omega.t) and sin (.omega.t), and does lowpass filtering to remove double-frequency components, noise, and adjacent interfering signals.
Ideally, the two outputs comprise the real component R.sub.0 at node 109 and imaginary component I.sub.0 at node 119 of the desired output signal x(t). If there is a phase imbalance in the quadrature oscillator, or an amplitude imbalance in the oscillator 103, mixers 105, 115 or lowpass filters 107, 117, then a distortion component proportional to x*(t), the complex conjugate of x(t), also appears. If the imbalance is molded (without loss of generality) by taking the oscillator outputs to be cos (.omega.t) at 105 and a sin (.omega.t+.phi.) at 115, the outputs at node 121 (after the imaginary part I.sub.0 at node 119 is scaled at 111 by j=.sqroot.-1 and that product added at 113 to the real part R.sub.0 at node 109) is EQU R.sub.0 +jI.sub.0 =x(t){1+ae.sup.-j.phi. }/2+x*(t){1-ae.sup.j.phi. }/2.
When a=1 and .phi.=0, there is no distortion. Otherwise there is an x*(t) distortion term with distortion-to-signal power ratio of approximately D/S=.vertline.1-ae.sup.j.phi. .vertline..sup.2 /4. As an example, to achieve at least a 40 dB D/S ratio, `a` must be between 0.98 and 1.02, and the phase error `.phi.` must be less than 1.15 degrees.
The distortion created by this imbalance can cause audio distortion in FM reception, and a bit error rate floor in digital systems. These problems might be especially severe, for example, in FM stereo applications or with very highly spectral efficient digital modulation such as 256 Quadrature AM. An even more stringent application in terms of balancing requirements is the FDM (Frequency-Division-Multiplexing) channel bank, where the input signal consists of a number of narrow-band signals which are to be separated and demodulated. The spectrum of the x*(t) interference term is a frequency-inverted version of the desired x(t) spectrum, scaled by the D/S ratio. If the narrow-band signals exhibit a dynamic range on the order of at least the D/S ratio, then reception can be severely degraded. A large dynamic range can result from frequency-selective fading of a single FDM signal arriving from one transmitter, or an FDM signal comprising a number of narrow-band signals arriving from different transmitters.
To date, quadrature balance is best achieved by implementing the zero-IF downconverter on a single integrated circuit, so that the parameters affecting the oscillator phasing and the gain matching between real and imaginary paths may be accurately controlled. Good circuit design can result in D/S ratios of 30 dB or so, which is inadequate for some applications, such as those indicated above. Therefore, the need exists for a way to improve quadrature balance.